1. Field of the Invention
The present invention relates to a focus state detection apparatus, and more particularly to a focus state detection apparatus suitable for a camera.
2. Description of the Prior Art
Various focus state detection apparatus have been proposed. They are classified into one of a defocus system and a deviation system. In the former system, when an image of an object is projected onto a CCD or BBD sensor, a sharpness is maximum in an in-focus state and the image is not sharp in an out-of-focus state. A difference therebetween is electrically detected to determine the in-focus state. In the latter system, a relative deviation of the object image is detected by a pair of photoelectric elements, as disclosed in Japanese Patent Application Laid Open Nos. 118,019/1970 and 155,331/1970 and U.S. Pat. No. 4,185,191.
In those methods, however, when the object image is converted to the electrical signal by the sensor, a noise component which is periodically generated by the sensor or a processing circuit is introduced into the photoelectrically converted signal. As a result, a resulting focus detection signal includes an error and hence a high precision focus detection is not attained. The above problem may be somewhat resolved by preventing the introduction of the noise into the sensor or the signal processing circuit but it is difficult to completely prevent the introduction of the noise and the circuit is complex of an antinoise performance is to be enhanced. On the other hand, the noise component can be reduced by an appropriate signal processing algorithm but it is not a complete solution to the problem.